I. Field of the Invention
The present invention relates to communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for performing idle handoff in a multiple access communication system. In addition, the present invention relates to an improved method for assigning a traffic channel in a multiple access communication system. The invention also relates to a method for reducing the number of required handoffs which occur while a mobile station is queued and waiting for a traffic channel in a multiple access communications system.
II. Description of the Related Art
Typically, communications systems prohibit handoffs while the mobile station is in a system access state. The system access state is the state in which communications are initiated either by the mobile station by means of transmissions over an access channel or by a base station by means of transmissions over a paging channel. In the exemplary embodiment, messages are sent in accordance with a code division multiple access (CDMA) communication format, which is disclosed in detail in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters" and U.S. Pat. No. 5,103,459 entitled "System and Method for Generating Waveforms in a CDMA Cellular Telephone System", both of which are assigned to the assignee of the present invention and are incorporated by reference herein. The use of paging and access channels for call initiation is well known in the art and is detailed in TIA/EIA Interim Standard IS-95-A, entitled "Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System".
One of the characteristics of CDMA systems is that the same frequency is reused in every cell. Diversity combining is a method by which a receiver receiving signals carrying the same information, combines those signals which are propagated through different paths to provide an improved estimate of the transmitted signal. A receiver design for taking advantage of the diversity signals carrying the same information but traveling through different propagation paths or transmitted by different transmitters is described in detail in U.S. Pat. No. 5,109,390 entitled "Diversity Receiver in a CDMA Cellular Telephone System", assigned to the assignee of the present invention and incorporated by reference herein.
Soft hand-off is a method by which a mobile station moving from one cell into another receives information from the base stations serving the two or more cells of the boundary area as long as the mobile station is located near the boundary. The signals which are sent by the base stations are combined in the receiver of the mobile station by the diversity combining method mentioned above. A method and system for providing soft hand-off in a CDMA communication system, where a plurality of base stations are in communication with a mobile station at or near cell boundaries is disclosed in U.S. Pat. No. 5,101,501 entitled "Method and System for Providing a Soft Handoff in a CDMA Cellular Telephone System", and U.S. Pat. No. 5,267,261, entitled "Mobile Station Assisted Soft Handoff in a CDMA Cellular Communication System," both of which are assigned to the assignee of the present invention and incorporated by reference herein. Hard handoff is, in contrast to soft handoff, where a mobile station passing from one cell to another is dropped by the cell being exited prior to being picked up by the cell being entered.
The use of the same frequency in every cell and the use of soft handoff results in high CDMA system capacity. The reuse of the same frequency in the neighboring cell causes rather rapid changes in the forward link signal to noise ratio near cell boundaries. This is because the cell being received by the mobile station may fade and the neighboring cell may increase in strength (anti-fade).
Generally, when the mobile station is receiving two cells, the received traffic channel energy per spreading chip to total spectral noise density for the signal transmitted by cell 1 is given by equation (1) below: ##EQU1## And the received traffic channel energy per spreading chip to total spectral noise density for the signal transmitted by cell 2 is given by equation (2) below: ##EQU2## where in equations (1) and (2)
I.sub.oc is the total thermal noise, ##EQU3## are the fraction of traffic channel power transmitted by cell 1 and cell 2 respectively, and
I.sub.or1, I.sub.or2 are the fraction of traffic channel power received at the mobile station from cell 1 and cell 2 respectively.
Consider I.sub.oc to be small relative to I.sub.or1 and I.sub.or2. When cell 1 fades relative to cell 2 I.sub.or1 becomes small relative to I.sub.or2 (the ratio becomes large). ##EQU4## Thus, ##EQU5## becomes small. If the mobile station is not in soft handoff, then this change in signal to noise ratio can cause problems. However, if the mobile station is in soft handoff with the neighboring cell, then the change in signal to noise ratio is not a problem because the mobile station is performing diversity combining of the forward traffic channels from both cells. While the first path given by ##EQU6## becomes small, the second path given by ##EQU7## becomes large. Thus, fading by one cell increases the signal to noise ratio from the other cell.
Paging is a method of sending information to a mobile station indicating the initiation of mobile terminated service or to receive an indication of new overhead information. A method for initiating a base station initiated call is described in detail in U.S. Pat. No. 5,392,287, entitled "APPARATUS AND METHOD FOR REDUCING POWER CONSUMPTION IN A MOBILE COMMUNICATIONS RECEIVER" and in copending U.S. patent application Ser. No. 08/206,701, filed Mar. 7, 1994, U.S. Pat. No. 5,509,015 which is a continuation of U.S. Pat. No. 5,392,287, both of which are assigned to the assignee of the present invention and are incorporated by reference herein. The present invention is equally applicable to mobile station initiated calls, the method for which is described in detail in copending U.S. patent application Ser. No. 08/219,867, filed Jan. 18, 1996, U.S. Pat. No. 5,544,196 which is assigned to the assignee of the present invention and are incorporated by reference herein.
In a slotted paging system, a mobile station monitors the paging channel for a short predetermined interval of time and then does not monitor the paging channel again until the next predetermined time interval. In IS-95-A this method of periodically monitoring the paging channel is called slotted mode and the mobile station may monitor the paging channel for 80 ms. every 1.28 seconds. The period between monitoring intervals can be made longer as desired by the user. Before each predesignated time slot in which a mobile station may be paged, the mobile station wakes up (becomes active) and resynchronizes or improves its synchronization with the base station. The mobile station then monitors for pages or other messages in the slot. After some interval, the mobile station can become inactive and not monitor the paging channel until just before the next assigned slot.
Prior to the time when a mobile station is actively communicating traffic information with the mobile communication system and after the time when the mobile station has achieved timing synchronization with the communication system, the mobile station is in a state referred to as the idle state. In the idle state, the mobile station can receive messages, receive an incoming call, initiate a call, initiate registration, or initiate message transmission. When in the mobile station idle state, IS-95-A permits the mobile station to perform an idle handoff at any time other than the interval that the mobile station is required to be monitoring its assigned slot.
However, when the mobile station originates a call or receives a page, the mobile station enters the system access state to send an origination message or a page response message. While in the system access state, an IS-95-A mobile station does not operate in the slotted mode. This is called non-slotted operation. Specifically, the mobile station continually monitors the paging channel until directed by the base station to a different state or an error condition occurs permitting the mobile station to exit the system access state. The exemplary embodiment will be described in the context of the origination operation and origination message, but the concepts directly apply to the mobile terminated call process and page response message. After the mobile station sends the origination message and receives an acknowledgment, the mobile station waits for a channel assignment message, which indicates upon which channel traffic communications from the base station to the mobile station will be conducted.
Upon receipt of the channel assignment message, the mobile station tunes to the allocated traffic channel, receives information on the forward traffic channel, and begins to transmit on the reverse traffic channel. The forward traffic channel is the channel upon which information from the base station to the mobile station is sent and the reverse traffic channel is the traffic channel upon which information from the mobile station to the base station is sent.
The interval between the time that the mobile station sends the origination message and the time at which the mobile station receives the channel assignment message depends upon the implementation of the individual infrastructure vendor. It can range from less than one-half of a second to several seconds. Until the time that the mobile station receives the channel assignment message, the mobile station is in the system access state.
The paging channel typically does not support soft handoff. Thus the issues of fading previously described occur. These are typically counteracted by having the radiated power of the paging channel higher than the traffic channel. Since one paging channel can handle the call origination and termination of many traffic channels, the loss in capacity by this higher power is minimal. In order to support soft handoff on the paging channel, the system would essentially have to send the same information on the paging channel in all cells, thus dramatically reducing the overall capacity of the paging channel.
While in the idle state, the mobile station is permitted to perform handoffs. Typically, the mobile station performs a handoff whenever the received signal level from one cell gets sufficiently above another cell. This idle handoff is typically done before the mobile station begins monitoring the slot. However, there can be cases in which the mobile station is unable to choose the correct cell before the slot begins and the mobile station must continue to monitor the existing cell. While in the system access state, the mobile station is not permitted to perform idle handoffs.
However, when the mobile station is in the system access state there can be cases in which the change in signal to noise ratio, ##EQU8## changes so fast that the message error rate becomes so high that mobile station cannot correctly receive the signaling messages sent on the paging channel. As a result, the mobile station may not receive the channel assignment message. This means that the call origination was not successful. IS-95-A permits the mobile station to exit the system access state and return to the mobile station idle state, if it has not received any paging channel messages for one second. This means that the mobile station does not receive the channel assignment message and the call origination was unsuccessful.
A similar problem exists when the mobile station is first assigned to the traffic channel. IS-95-A permits only a single base station to be assigned to the mobile station. If another cell is strong or becomes stronger, the mobile station may not be able to receive the forward traffic channel successfully. As a result, the call may drop. The problem is that the mobile station is assigned to a traffic channel with a single active set member and is not in soft handoff.
Under IS-95-A, in order for the mobile station to enter into soft handoff, the following steps must occur. First, the mobile station detects that the pilot of another base station is above a predetermined energy threshold value. Second, the mobile station sends a pilot strength measurement message. Third, the infrastructure sets up the handoff, and the infrastructure sends the handoff direction message to the mobile station. Depending upon the circumstances and the implementation, this may take from a few hundred milliseconds to considerably more than one second.
Thus, although soft handoff is generally supported in IS-95-A systems. However, soft handoff is not supported when the mobile station is in the system access state. Thus, there is a need for a system which permits soft handoff while the mobile station is in the system access state to provide increased reliability in the system access process and other benefits.